X-ray analysis of the structure of the wall of Valonia ventricosa .—I

Abstract

In searching for material built from cellulose which might, by the application of X-ray analytical methods, throw further light on the nature of the cellulose lattice and its orientation in biological structures, Sponsler (1930) examined the green alga, Valonia ventricosa , which grows in the form of a single-celled hollow sphere often as much as 2 cm. in diameter. From the X-ray Photographs which he obtained, he was able to show, not only that the cell-wall is composed of crystallites of cellulose which for such a substance must be considered as relatively well-developed, but also that these crytallities are so laid down in the wall that there is a preferential orientation of the crystal lattice with respect to the surface. The plane of "spacing" 6·10 Å. U. lies roughly parallel to the wall, while the plane, of "spacing" 5·33 Å. U. lies roughly perpendicular to the wall. On the other hand, Sponsler (1931) concluded that there is no further selective orientation of the cellulose crystallites, but that, "in attempting to determine, in Valonia , the position of the remaining axis, the (cellulose) chain axis, the diffraction lines which indicated the presence of the chains showed that a random distribution of the chains occurred around a radial reference line of the wall." By the courtesy of the same worker, the actual specimens on which these observations were made were soon afterwards submitted to an optical examination in polarised light by R. D. Preston (1931) of the Botany Department of the University of Leeds. His results appeared to offer a striking confirmation of those of Sponsler, for he found that the wall was divided approximately into areas forming a mosaic, each area having its own interference colour, different from that of its surroundings. The average size of the areas was about 1·5 × 10 -4 sq. cm., and the extinction directions in general varied considerably in passing from one area to another. Preston assumed, in accordance with what appears to be a well-established, but as the sequal shows erroneous, custom, that the extinction directions in any area were parallel and perpendicular to the directions of the cellulose chains in that area, and therefore concluded, since many interference patches would be irradiated simultaneously by an X-ray beam such as Sponsler used, that the effect of random orientation described by the latter was in perfect agreement with his own findings. The fact, however, that Sponsler had used for his investigations, not single pieces of cell-wall, but blocks made up of a large number of such pieces, suggested that there might be a fallacy in this argument, even though the blocks in question were so build up that meridians running from the hold-fast of the spherical cell were as far as possible parallel. In spite of the fact that the optical examination suggested the contrary, it was still possible that the X-ray diffraction effects had been produced by the accumulation, in the large mass of material used, of relatively slow variations of orientation distributed over the whole surface of the cell-wall. It seemed, therefore, that a more promising line of attack would be to take X-ray photographs of single pieces of wall and, if possible, limit the X-ray beam each time to a single interference patch which had been previously defined with the aid of the polarising microscope.